root cache building? Newb Q

[Gee308]

Hi, I'm new to DNS
I just set up OpenBSD 2.7 with BIND.  I used the default file, /etc/named.boot
to startup my DNS except I changed the root.cache to a local db file of
ftp://ftp.rs.intercom.net/domain/named.root.  The DNS works fine.  Will my DNS
database grow and be saved as it looks up new domains?  If not, where can I get
a db file that has many/most/all of the dns entries. 
 Also, will I get faster lookups with my own DNS server that runs on a 486
33mhz with 16mb of RAM machine or use the regular ISP provided DNS?  I know
that these questions are probably stupid, but im very new to this.  I read over
the man pages, and a few DNS sites, but it was way too much information for me
to store at one time(I just set up sendmail for the first time that night).
Thanks for the help in advance.
Please reply to me at
gee308 at mediaone.net
http://starcraftlinux.eyep.net